Modern clinical and research analysis techniques require careful and controlled identification and separation of the various types of cells found in the blood, for example white cells, red cells, and platelets. Typically, small blood samples are taken, appropriately diluted and subjected to various reagents and/or dyes, and the diluted subsamples are appropriately analyzed. Often, histograms are obtained which set forth distributions of various cells as a function of specified parameters, such as volume. Based upon a knowledge of the nature and characteristics of the respective cells, these histograms may often be effectively correlated with the presence of cells of various types.
One challenging problem in the design and production of effective blood analysis instruments has been the discrimination, identification, and analysis of platelets. In particular, by virtue of the size, size distribution, and overall number of platelets per unit volume, substantial discrimination difficulties have been engendered. For example, the smallest of platelets often are confused with particles, microbubbles, or other spurious elements in the diluted blood sample under investigation. The largest of platelets often have a similar volume to that of small red cells, for example in the range of thirty to forty cubic microns. Moreover, the number of platelets per unit volume of blood normally is much smaller than the number of red cells in the same volume, such that if the red cells possess even a normal size distribution, the number of small red cells may be quite comparable to the number of platelets in the sample. Dilution steps generally have little effect on these problems, since red cells, white cells, platelets, and the like are conventional, diluted in the same ratios. In fact, the normal consequence of dilution is that fewer cells overall are available for analysis, thereby tending to increase the severity of the problem.
The principal, and heretofore most generally successful, prior art approach to platelet discrimination and analysis has relied upon principles relating to electrical conductivity or resistance. In accordance with this approach, a pair of electrolyte tanks are maintained adjacent to one another, separated only by an electrically insulating wall. A positive electrode is inserted into one tank, and negative electrode into the other. A small orifice, typically in the range of 50-75 microns in diameter, penetrates the wall, thereby interconnecting the two tanks of electrolyte. A blood sample is inserted into one tank, and by maintenance of flow, passes from one to the other through the orifice. The resistance of the orifice is carefully monitored, and this resistance is changed as cells pass through the orifice, based on different conductivities (or resistances) of different types of cell. Generally, the resistance change as a cell passes through the orifice is a function of the volume and the shape of the cell.
This prior art approach is satisfactory for normal red cells and normal platelets, but encounters a rather severe discrimination breakdown with respect to large platelets and small red cells, which possess comparable volumetric parameters. Such problems are further exacerbated by the fact that the actual number of small red cells in a normal sample is comparable to the number of platelets in the sample. Conventional attempts to correct these problems, and to distinguish between small red cells and large platelets, utilize mathematical correction, generally based on the assumption that red cells have a known volume distribution whereas platelets have a different, but known volume distribution. On this basis, algorithms have been derived to correct the actual data and to depict a calculated discrimination between red cells and platelets. These algorithms typically are adequate for non-pathological blood samples, but often are inaccurate for pathological blood samples, or samples which have recently been subject to transfusions.
These prior art systems also encounter difficulties with respect to accurate detection of the smallest of platelets. That is, the prior art electrical resistance type of systems encounter problems with respect to electronic noise, small spurious particles at or near the orifice, or acoustic vibration in the electrolytes. Algorithms have been developed in order to attempt to correct for these difficulties, but it is evident that these algorithms are at best in the nature of estimates, and in fact cannot adequately account universally for particulate, vibratory, and the like difficulties which may be attendant to the installation of the instrument or to the procedures utilized by the technician, and not to any physical or physiologically related factors